The present invention relates to communications systems and methods, and more particularly, to apparatus and methods for synchronization with modulated signals.
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have been long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a typical terrestrial cellular radiotelephone communication system 20. The cellular radiotelephone system 20 may include one or more radiotelephones (terminals) 22, communicating with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular network may include hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 22 and the MTSO 28, by way of the base stations 26 serving the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. A control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the cellular network 20, a duplex radio communication link may be effected between two mobile terminals 22 or between a mobile terminal 22 and a landline telephone user 32 through a public switched telephone network (PSTN) 34. The function of a base station 26 is to handle radio communication between a cell 24 and mobile terminals 22. In this capacity, a base station 26 functions as a relay station for data and voice signals.
As illustrated in FIG. 2, a satellite 42 may be employed to perform similar functions to those performed by a conventional terrestrial base station, for example, to serve areas in which population is sparsely distributed or which have rugged topography that tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and terminals 23. The satellite conveys radiotelephone communications over duplex links 46 to terminals 23 and an earth station 44. The earth station 44 may in turn be connected to a public switched telephone network 34, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system 40 may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams 48, each serving distinct geographical coverage areas 50 in the system""s service region. The coverage areas 50 serve a similar function to the cells 24 of the terrestrial cellular system 20 of FIG. 1.
Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels. As a practical matter well known to those skilled in the art, radiotelephone communications signals, being modulated waveforms, typically are communicated over predetermined frequency bands in a spectrum of carrier frequencies. In a typical FDMA system, each of these discrete frequency bands serves as a channel over which cellular radiotelephones communicate with a cell, through the base station or satellite serving the cell.
The limitations on the available frequency spectrum present several challenges as the number of subscribers increases. Increasing the number of subscribers in a cellular radiotelephone system requires more efficient utilization of the limited available frequency spectrum in order to provide more total channels while maintaining communications quality. This challenge is heightened because subscribers may not be uniformly distributed among cells in the system. More channels may be needed for particular cells to handle potentially higher local subscriber densities at any given time. For example, a cell in an urban area might conceivably contain hundreds or thousands of subscribers at any one time, easily exhausting the number of channels available in the cell.
For these reasons, conventional cellular systems employ frequency reuse to increase potential channel capacity in each cell and increase spectral efficiency. Frequency reuse involves allocating frequency bands to each cell, with cells employing the same frequencies geographically separated to allow radiotelephones in different cells to simultaneously use the same frequency without interfering with each other. By so doing, many thousands of subscribers may be served by a system having only several hundred allocated frequency bands.
Another technique which can further increase channel capacity and spectral efficiency is the use of time division multiple access (TDMA). A TDMA system may be implemented by subdividing the frequency bands employed in conventional FDMA systems into sequential time slots. Communications over a frequency band typically occur on a repetitive TDMA frame structure that includes a plurality of time slots. Examples of systems employing TDMA are those conforming to the dual analog/digital IS-54B standard employed in the United States, in which each of the frequency bands of the traditional analog cellular spectrum are subdivided into 3 time slots, and systems conforming to the GSM standard, which divides each of a plurality of frequency bands into 8 time slots. In these TDMA systems, each user communicates with the base station using bursts of digital data transmitted during the user""s assigned time slots.
Instead of or in addition to FDMA and TDMA techniques, wireless communications systems may employ xe2x80x9cspread spectrumxe2x80x9d or code division multiple access (CDMA) techniques. In a system employing spread spectrum techniques, a channel may be defined by modulating a data-modulated carrier signal by a unique spreading code, i.e., a code that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates. Data may be recovered from the transmitted signal by demodulating the signal using the same spreading code. Because the transmitted signal is spread across a wide bandwidth, spread spectrum communications can be less vulnerable to coherent noise sources which might xe2x80x9cjamxe2x80x9d other communications signals. The use of unique spreading codes for channels allows several users to effectively share the same bandwidth.
Conventional spread-spectrum communications systems commonly use so-called xe2x80x9cdirect sequencexe2x80x9d spread spectrum modulation. In direct sequence modulation, a data-modulated carrier is directly modulated by a spreading code or sequence before being transmitted in a communications medium, e.g., an air interface. The spreading code typically includes a sequence of xe2x80x9cchipsxe2x80x9d occurring at a chip rate that typically is much higher than the bit rate of the data being transmitted.
A direct sequence spread spectrum receiver typically includes a local sequence generator that locally produces a replica of a spreading sequence. This locally generated sequence is used to recover information from a transmitted spread spectrum signal that is modulated according to the same spreading sequence. Before information in a transmitted signal can be recovered, however, the locally generated spreading sequence typically must be synchronized with the spreading sequence that modulates the transmitted signal.
In conventional spread spectrum systems, synchronization is commonly achieved by transmitting a known pseudo-random noise (PN) sequence that a receiving unit can acquire and use to synchronize its de-spreading operations. A base station may broadcast, for example, a xe2x80x9cpilot signalxe2x80x9d comprising a fixed carrier modulated by a known PN sequence or a sequence of xe2x80x9cpilot symbolsxe2x80x9d embedded at known locations in a transmitted data stream, with the transmitted pilot signal or pilot symbols being received by a mobile terminal and used to synchronize its de-spreading operations with the spreading operations of the base station.
As illustrated in FIG. 3, a radio communications signal 305 is received at an antenna 310 of a receiver 300 and processed in a radio frequency (RF) section 320 to produce baseband samples. Baseband samples corresponding to a received pilot signal are processed in a sequence acquisition and tracking circuit 340 to acquire and synchronize to the transmitted PN sequence. The synchronization information thereby obtained is used to synchronize spread spectrum demodulation in a spread spectrum demodulator 330.
Synchronization with a pilot signal or series of pilot symbols typically includes an initial acquisition phase during which a receiver searches for a known PN sequence in a received communications signal, followed by a tracking phase that achieves finer synchronization once the known PN sequence is detected. Commonly, acquisition is achieved by using a xe2x80x9csliding correlatorxe2x80x9d that computes a series of correlations between a sequence of samples of a received signal and the known PN sequence by xe2x80x9cslidingxe2x80x9d the sample sequence past all or a portion of the known PN sequence. The sliding correlator typically produces a correlation output that exhibits a pronounced peak value when correlation between the received signal sample sequence and the known PN sequence is high, i.e., when the sample sequence and known sequences are in phase.
It is generally desirable that this search process be performed as accurately and as quickly as possible. Although a sliding correlator that provides full-period correlation between the received signal samples and the known PN sequence may be desirable from the standpoint of accuracy, full period correlation may be impractical because the PN code used for the pilot signal of a spread spectrum system may be on the order of thousands of bits in length. A correlator designed to perform full period correlation on a code of such length would generally be slow, complex and power-inefficient.
For this reason, many practical sliding correlators correlate received signal samples with only a portion of the known PN sequence. Such partial-sequence correlation generally does not have well-known, structured correlation properties, and performance is generally dependent on the portion of the PN sequence that is used in the sliding correlator.
In light of the foregoing, it is an object of the present invention to provide receiver synchronizing methods and apparatus that can produce improved acquisition performance.
It is another object of the present invention to provide partial-sequence correlation methods and apparatus that can provide correlation outputs with improved discrimination between in-phase and out-of-phase conditions.
These and other objects, features and advantages are provided in a receiver by apparatus and methods in which a received signal, e.g., a pilot signal modulated according to a known-modulation sequence, is correlated with a subsequence of the modulation sequence that is selected based on a correlation between the subsequence and the modulation sequence. Preferably, the selected subsequence has an optimal, e.g., minimized, out-of-phase correlation with the modulation sequence in comparison to other subsequences of the modulation sequence. According to one aspect of the present invention, the selected subsequence is a subsequence that meets a xe2x80x9cmin-maxxe2x80x9d criterion, i.e., a subsequence having a minimum maximum out-of-phase component of the correlation with the modulation sequence. The maximum out-of-phase component may represent a single maximum out-of-phase correlation value or a sum of largest out-of-phase correlation values. According to another aspect of the present invention, the selected subsequence is a subsequence that meets a xe2x80x9cmax-powerxe2x80x9d criterion, i.e., a subsequence having a minimum aggregate out-of-phase correlation with the modulation sequence. According to other aspects, combined maximum and aggregate or minimized xe2x80x9cnearest neighborxe2x80x9d criteria are used.
Use of a correlation subsequence that is selected based on a relative correlation property of the subsequence can improve the accuracy and reliability of partial sequence synchronization. For example, a subsequence meeting a xe2x80x9cmin-maxxe2x80x9d criterion may be used to reduce worst case false synchronization detection, while a subsequence meeting a xe2x80x9cmax-powerxe2x80x9d criterion may be used to reduce the average false alarm probability over all out-of-phase conditions.
In particular, according to the present invention a receiver in a communications system is operated by receiving a radio communications signal transmitted by a station of the communications system and generating a correlation output representing a correlation of the received radio communications signal and a subsequence of a modulation sequence that is selected based on a correlation between the selected subsequence and the modulation sequence. The receiver is then synchronized based on the generated correlation output. The selected subsequence preferably has an optimal out-of-phase correlation with the modulation sequence.
According to an aspect of the present invention, the selected subsequence has a minimum out of phase correlation with the modulation sequence in comparison to other subsequences of the modulation sequence. For example, the selected subsequence may have a xe2x80x9cminimum maximumxe2x80x9d out-of-phase correlation value, a xe2x80x9cminimum aggregatexe2x80x9d out-of-phase correlation metric, or a combination thereof associated therewith.
In one embodiment of the present invention, the communications system comprises a spread spectrum communications system that transmits a communications signal modulated according to a predetermined pseudo-noise (PN) sequence. The transmitted communications signal is received and a correlation output is generated representing a correlation of the received communications signal and a subsequence of the PN sequence selected is generated based on a correlation of the selected subsequence with the PN sequence. The receiver is synchronized based on the generated correlation output.
According to another aspect of the present invention, a modulation sequence encoded in a communications signal is detected by correlating the communications signal with a selected subsequence of the modulation sequence to generate a correlation output representing a measure of correlation between the communications signal and the modulation sequence, wherein the selected subsequence is selected based on a correlation between the selected subsequence and the modulation sequence. The selected subsequence preferably has an optimal out-of-phase correlation with the modulation sequence.
According to yet another aspect of the present invention, in a receiver for receiving a communications signal modulated according to a modulation sequence, an apparatus comprises a correlator operative to correlate the communications signal with a selected subsequence of the modulation sequence to generate a correlation output representing a measure of correlation between the communications signal and the modulation sequence, wherein the selected subsequence is selected based on a correlation between the selected subsequence and the modulation sequence. The selected subsequence preferably has an optimal out-of-phase correlation with the modulation sequence. The apparatus may further comprise means for receiving a communications signal modulated according to a predetermined pseudo-noise (PN) sequence, and the correlator may be coupled to the means for receiving and operative to generate a correlation output representing a correlation of the received communications signal and a selected subsequence of the PN sequence, wherein the selected subsequence is selected based on a correlation of the subsequence with the PN sequence. The apparatus may further comprise means for synchronizing the receiver based on the generated correlation output. Improved communications performance may thereby be provided.